Plants are boring. At least that is what I—as well as countless others—thought in school. Animals seemed far more exciting than studying plants. In hindsight, I wonder why I didn’t find plants interesting. One of the reasons was that I couldn’t see plants moving—with the exception of ‘touch-me-nots’ that rapidly fold inward upon touching—and they aren’t cute and cuddly as mammals are. Later, when I learned that plants produce their own sugars using water, carbon dioxide, and sunlight—a phenomenon we know as photosynthesis and achieved by only a few other life forms— I got a little interested.
But what really piqued my curiosity and captivated me was when I learned that some unusual plants go a step further: they have evolved to ‘eat meat’—insects in particular. We normally expect insects to eat plants, which in turn are preyed on by larger animals, as the food web goes. But when the roles are reversed, it is harder for us to digest that plants can actually play the role of predators.
Carnivorous plants are quirks of evolution: These sinister predators grab nutrients from insects by attracting, capturing, killing, digesting, and absorbing them—a behavior we normally associate with the animal kingdom. Simply killing prey doesn’t classify as carnivorous; plants have to absorb the digested nutrients and use them for their own growth, otherwise they are just ‘murderous’ plants—those that kill without deriving any nutritional benefit from corpses.
Plants have existed long before any of the animals we see on land today, forming the base of the food chain, upon which all life depends. For millions of years, plants have been dining on insects, yet they were only studied in detail by Darwin, who was mesmerized by these plants. He outlined detailed observations of the methods of prey capture employed by carnivorous plants, and their evolution in his pioneering book “Insectivorous Plants”, where he called the Venus flytrap as “one of the most wonderful [plants] in the world.” For decades, people have been fascinated by meat-eating plants, notably the Venus flytrap and pitcher plants. The rapidly snapping jaws of the former in response to prey reveals the dark side of plants, and proves that even plants can turn rogue. But why did they evolve in the first place? Why would a plant resort to ‘eating’ insects?
While plants procure carbon from the atmosphere, other nutrients, such as nitrogen, phosphorus, and sulfur are obtained from the soil, usually with the help of bacteria, which convert these nutrients, such as ammonia from the atmosphere, into nitrates, a form that plants can readily use. Nitrogen is an essential component of chlorophyll, which in turn is necessary for photosynthesis. But soils in areas such as bogs, swamps, and marshes are devoid of these essential nutrients—although there is plenty of sunlight and water. How then can plants growing in these areas survive?
They have come up with an astonishing—and bizarre—solution: obtaining nutrients through carnivory. It is in these nutrient-poor environments where most carnivorous plants thrive, and can be spotted, though they are rare. Scientists believe that carnivory evolved among plants mainly to fulfill their requirements for nitrogen and phosphorus, which are deficient in their environments. Some plants also use carbon from their prey, especially when light and carbon dioxide are limited. (pic of plant growing in a bog)
Scientists estimate that carnivory evolved independently at least six times in five different orders of flowering plants. And carnivorous plants are amazingly diverse: more than 700 species exist—and some have been extinct already. Despite having evolved independently, they share remarkable similarities in their prey-trapping and digesting mechanisms.
Carnivorous plants have evolved a variety of mechanisms to trap prey. Traps fall into five broad types: pitfall traps, flypaper or sticky traps, snap traps, suction traps, and eel-traps. Pitfall traps are passive traps because they do not physically move while trapping prey. On the other hand, the other four traps are active, as they move while trapping prey (but some flypaper traps are passive). Each trap is thought to have evolved from hairy leaves turning into specialized leaf traps.
Pitfall traps are comprised of pitchers, where the tropical pitcher plants from the genus Nepenthes, form the largest group represented by more than 110 species located in subtropical regions of Asia where the climate is warm and rainfall is high. Nepenthes pitchers are majestically diverse in size, shape, and color; the largest pitchers can be over 40cm in height, such as the famous giant species Nepenthes rajah, which is only found in mountains of Malaysian Borneo, while the smallest are only a few centimeters in height. The genus continues to diversify with most of the species found exclusively in the islands of Borneo, Sumatra, and the Philippines.
The second and third largest groups are from the genus Heliamphora (Sun Pitcher plant) and Sarracenia (American Pitcher plant). The former contains eighteen species only found in the mountainous regions of Guiana where it is cold and foggy, whereas the latter consists of nine species found in southeast America, except for one which is also found in the north of the country. Another species native to America is Darlingtonia californica, also known as the Cobra plant, which as the name suggests is found in northern California. The pitchers of both Sarracenia and Heliamphora resemble inwardly rolled leaves fused together.
One pitcher plant species Cephalotus follicularis (Albany pitcher) is native to southwest Australia. It is often found in coastal areas, swamps, and near streams. When exposed to sunlight, the pitchers turn dark red but in the shade, they retain their green color.
A Fatal Attraction
Innocuous-looking pitchers resembling jugs hang from tendrils attached to leaves. They have a flap on the top serving as a lid to shield them from overflowing and washing out prey during rainfall. These passive traps tantalize prey such as foraging or crawling bugs as well as flies by secreting sugary nectar on the inner rims of the pitcher, emitting sweet aromas, and some even resort to glowing under ultraviolet light. But this attraction is insidious: once on the rim, unwary victims lose their foothold and end up stumbling into the pitcher.
Inside, they find it difficult to climb out as waxy crystals or downward pointing hairs line the surface of the inner pitcher wall in some species. Wax crystals are rough, reducing the surface area that insects’ adhesive pads can cling on to. Their claws become clogged by the crystals causing them to lose their grip; any attempt to climb out often proves futile. Essentially, they have tripped into their own graves.
Glands at the bottom of the pitcher secrete a pool of fluid containing a cocktail of digestive enzymes that dissolve the carcasses into goo. Finally, the plant extracts their nutrients, which are then absorbed by the glands and used by the plant. In many species, the digestive fluid is both viscous and elastic, kind of like the consistency of honey, and these properties help in retaining prey by preventing them from resurrecting and ensuring that they die by drowning in the fluid. During its lifetime, a single pitcher can devour thousands of insects.
Nepenthes plants produce two types of pitchers: lower and upper. When young, they produce pitchers that hang low near the ground; as the plants mature, they produce upper pitchers.
Slippery and Slimy
The wax lining and the viscoelastic fluids are adapted to different types of prey. Ants are effectively captured by the wax lining, whereas the viscoelastic fluid is more efficient at retaining flies in addition to ants. Researchers observed that flies that were not wetted by the fluid were sometimes lucky enough to escape by climbing up the pitcher wall and taking off, but once wetted by the sticky fluid they had little chance of escaping; the more they tried to resist the gooey fluid, the more they got entangled in it. In higher elevations, where there are fewer ants and more flying insects, Nepenthes species produce stickier or more viscoelastic digestive fluids than their lowland counterparts.
Even within the same plant, many Nepenthes lower pitchers are waxy targeting mainly ants. In contrast, the upper pitchers produce highly viscoelastic fluid which is efficient at trapping flies. The fluid is so viscoelastic that even when it is diluted in 95 percent water, it remains sticky enough to capture insects.
In addition to the wax crystals inside the pitcher, the rims play crucial roles in trapping prey. During rain or condensation, the collar-shaped rim, known as the peristome, becomes wet as the raindrops spread forming a fine layer. This makes it extremely slippery for prey that end up easily losing their foothold and falling inside the pitcher. In contrast, the wax crystals on the inside do not depend on water; they are always active.
Nectar is not only a ploy for attracting prey but is also involved in capturing prey. Secreted from the inner peristomes, it absorbs water from the air rendering it even more slippery and thereby increasing their prey-trapping efficiency. As a result, ants can probably get away with the nectar during dry periods in the day; and in turn, they recruit more ants to the site. But, when they visit the pitchers for a sweet midnight snack at night—when it is wetter—they may not be so lucky.
Climate influences the strategy plants employ for trapping prey, whether it is through the peristome, wax, or viscoelastic fluids. Since peristomes rely on ambient humidity for effective trapping, species with larger peristomes and lacking a waxy coating inside are confined to equatorial perhumid regions such as western Borneo and western Sumatra. In contrast, species with small peristomes and wax layers present were found in a wider, more seasonal climate range, both above and below the equator. Viscoelastic species have the most restricted distribution, mostly confined to perhumid regions. This is consistent with reports of many highly viscoelastic species found in the mountains of Southeast Asian countries that are located near the equator.
Nepenthes species have evolved to specialize in either of these trapping mechanisms, although some species employing both strategies exist. Species that lack wax crystals compensate by boasting larger and more inward-sloping peristomes. Ancestral Nepenthes pitchers were likely small with fully developed wax crystal layers, which were independently lost at least eight times over the course of the genus’ evolution.
Several Nepenthes species can be seen growing in the same area in the islands of Borneo and Sumatra, yet they have evolved different trapping structures, suggesting they probably avoid competition by specializing in specific types of prey.
Floundering in the fluid
Pitchers have to fend off microbes that can hijack the viscoelastic fluid and steal nutrients extracted from prey. Fluid collected from unopened Nepenthes pitchers was analyzed and found to be free of bacteria. Even when bacteria and baker’s yeast were added into the pitchers, bacteria died, whereas the yeast cells survived, but they did not grow inside the pitchers, presumably due to lack of essential nutrients. Upon addition of the microbes, pitchers rapidly produced defensive proteins and antimicrobial compounds, such as plumbagin, which is also a toxin, preventing bacteria from establishing themselves inside the pitcher.
Another study found that the roots and the top waxy layers of Nepenthes khasiana pitchers, found in the Indian subcontinent, produced plumbagin in large quantities upon mimicking prey capture by feeding it with chitin, a component of insect exoskeleton into the pitchers. The researchers surmise that plumbagin may serve a dual function: fight against fungal infection in the pitcher tissues and help in trapping prey by exerting an anesthetic effect in which prey become ‘paralyzed’ and lose their normal escape behaviors.
Scientists have identified another unique and novel method of capturing prey adopted by Nepenthes gracilis: through ‘flicking’ prey off its pitcher lid. The underside of the lid not only has wax crystals like the peristome, it also produces copious nectar to which ants are attracted. During rainfall, heavy raindrops effectively ‘flick’ prey such as ants, beetles, and flies off the lid, so that they fall directly inside the pitcher. When researchers applied an anti-slip coating on the underside of the lid, prey capture was lower, indicating that the lid plays an important role in capturing prey. The wax crystal surface of the underside of the lid is vastly different from the lining on the inside of the pitcher walls whose continuous structure resembles that of other Nepenthes species.
In another unusual method to attract prey, Nepenthes aristolochioides, uses light. The pitchers have a unique shape: Its mouth opens at the front, instead of at the top, and the back of the pitcher wall is dome-shaped. Sunlight passes through the dome, which is more translucent at the rear, making the interior appear brighter, against its brown mottles when peering in from the outside. Attracted to the light, flies enter the mouth, only to find themselves trapped inside. When scientists shaded the pitchers at the rear to block light from passing through, there were three-fold fewer flies caught compared with unshaded pitchers.
Recently, researchers discovered various Nepenthes species, some Sarracenia species, as well as the Venus flytrap, also use light as a lure, but more sophisticated than the previous example of ordinary sunlight passing through the pitcher walls. The peristomes of N. khasiana glow a vivid fluorescent blue under ultraviolet light (366nm). In some cases, even the insides of the pitcher tubes glowed blue—from the bottom until the level of the digestive fluid.
Suspecting that insects, particularly flying ones, which can perceive blue and green in normal light and can see in UV light, are drawn towards the blue light, researchers devised an experiment: They coated the peristomes with acetone, an ingredient in nail polish remover, to block the light and after ten days, they compared the prey caught with that of unmasked pitchers. Confirming their suspicions, the blue glow serves as a very efficient lure: There was a drastic drop in prey capture—mostly ants—for the masked pitchers compared with the ones that glowed blue. Insects, particularly ants, surely seemed to love the blue lights.
In Nepenthes, the pitcher fluid is produced by the plant in larger quantities; Sarracenia plants, on the other hand, produce smaller amounts of fluid. The pitchers of the latter rely on collected rainwater, which trap drowned prey. Fluid levels can reach up to 1.5 liters in some Nepenthes pitchers. Despite the acidic and antimicrobial properties of the fluid, they are bursting with activity, with some pitchers hosting an entire microcommunity of organisms! The inhabitants include a concoction of bacteria, fungi, rotifers, algae, arthropods, and vertebrates, which all thrive inside the pitcher by dining on the captured prey—thereby vying with the plant for nutrients.
While Nepenthes produces its own digestive enzymes and its fluids are both toxic and acidic, Sarracenia doesn’t produce as much, and rely mostly on bacteria for digesting prey. Consequently, Sarracenia hosts a large diversity of organisms that actually help it digest prey by producing enzymes. Sarracenia purpurea, commonly known as the purple pitcher plant and prevalent boggy, marshy areas of northeastern America, was found to host 165 species of organisms; in contrast Nepenthes ampullaria had only 59 species. These differences are due to the size and life span of pitchers as well as the properties of its digestive fluid as some fluid environments might be inhospitable for some bugs, but hospitable for others. In this way, plants are able to control, to some extent, the types of bugs that inhabit them. Researchers discovered the highest species diversity of fly larvae and protists in the pitcher fluids. The food web of organisms inside pitchers is complex.
Most of these organisms, however, are a boon to the plants: They secrete enzymes, degrade carcasses into smaller pieces, and excrete ammonium, the preferred form of nitrogen by Nepenthes, as well as phosphate compounds that the plant can absorb. Metriocnemus knabi, a tiny midge, lives at the bottom of the fluid in S. purpurea, breaking up drowned prey and speeding up digestion. Bacteria help release nutrients from the insects by decomposing the carcasses. Microalgae can compete with pitchers for nitrogen; but S. purpurea benefits from Wyeomyia smithii, a mosquito that breeds only in its pitchers, and prevents microalgae from colonizing them. It is the top predator in the pitcher. The mosquito larvae as well as fly larvae both feed on bacteria, thereby regulating the abundance and diversity of bacteria growing on prey, including those that help convert nitrogen from prey into a form the plant can readily uptake.
Recently, scientists discovered that the fluid-filled pitchers of N. ampullaria are the breeding place and nursery for Microhyla nepenthicola, one of the world’s smallest frogs, which are about ‘the size of a pea.’ It happily lays its eggs in the sides of the pitcher and the tadpoles hatch inside them.
Scientists have discovered that N. bicalcarata gets its nitrogen from the ants’ droppings. On average, almost half of the foliar nitrogen in the plant comes from ant waste. Compared with plants uninhabited by the ants, the occupied plants produced more abundant and larger leaves and pitchers. But the ants also help the plants in another way: They prevent parasites such as mosquito larvae from stealing prey by hunting and attacking them. Diving into the pitcher fluid, the ants aggressively grab and drag pupae from the pitcher fluid to the walls of the pitcher. This way they guard the plant from exploitation by intruders.
A video clip of the aggressive hunting behavior displayed by Camponotus schmitzi for mosquito pupae inside the pitcher fluid.
Another video of Camponotus schmitzi hunting fly larvae. You can even spot the tadpoles of Microhyla nepenthicola in the background.
Pitchers also have to contend with unwanted visitors. Some herbivores such as caterpillars damage the pitchers’ trapping ability by munching on the inner wall. Others such as geckos, crabs, and apes steal the pitchers’ catches. Some terrestrial spiders take advantage by sealing pitchers with their webs, so the pitchers are ineffective at catching prey. However, some larger trap visitors can help the plant by feeding on the caterpillars that damage the pitchers.
In rare cases, especially in bigger pitchers, prey catches have been bewilderingly large: Birds, rodents, lizards, and frogs have been trapped and devoured by pitcher plants, although many of them are likely to have fallen in accidentally. In an unusual case, a great tit was found lodged inside a pitcher in a nursery in the UK. It is believed the unfortunate bird had fallen in while stooping down to grab some insects floating in the pitcher.
Much to his amazement, a botanist had spotted a shrew skeleton and undigested fur inside a large Nepenthes attenboroughii pitcher in Mount Victoria in the Philippines.
Poop, Glorious Poop
In higher altitudes, however, prey is scarce. As a result, Nepenthes lowii, a species endemic to Borneo, has veered away from carnivory, resorting to an even stranger, yet innovative, method of obtaining nitrogen: by acting as a lavatory for a small mammal. While the lower pitchers look like those found in other Nepenthes plants targeting arthropod prey, the upper pitchers are distinct in shape and adaptations: the pitchers have a wide opening, with no waxy zone inside; the rim is narrow and the pitcher lids produce copious nectar and emit fruity odors. These adaptations are perfectly suited to attract tree shrew visitors, Tupaia montana, a species only found in Borneo, who are attracted to the pitcher lids for nectar. While enjoying the sweet treat, they end up defecating into the pitcher. And poop is exactly what the plant is after. Nitrogen from the feces contributes significantly to foliage, with foliar nitrogen levels in mature plants reaching as high as 100 percent.
Other Nepenthes species, such as N. rajah, a giant montane species in Borneo, also take advantage of this strategy. In an unexpected discovery, scientists noticed its pitchers are frequently visited by T. montana as well as another species only found in Borneo, the summit rat, Rattus baluensis. Both species feed on the sugary fluids secreted by the pitcher lids. T. montana visits the pitchers only during the day, whereas most of the visits by R. baluensis were observed at night. Although both species deposit feces at similar rates, N. rajah is likely to gain extra nutritional benefits from receiving deposits both during the day as well as at night. The more poop, the better!
Utilizing mammal droppings seems to be gaining traction among Nepenthes species located in higher altitudes. Like its counterparts, the aerial pitchers of N. rafflesiana elongate (also known as Nepenthes hemsleyana), growing in the peat swamps of Borneo also harvest poop, but from an unlikely mammal: bats. These pitchers produce low amounts of digestive fluid and insect-enticing odors. During the daytime, tiny Hardwicke’s woolly bats measuring less than six centimeters in length, often snuggle inside the narrow, slender pitchers. Shielded from predators, and protected from the rain and bright sun, the pitcher offers a safe and cozy home. In return for providing a comfortable roosting place, the plant benefits from the bat’s poop deposited into the pitchers. A third of total foliar nitrogen comes from bat poop. Looks like caves aren’t the only place you’ll find bats.
In another bizarre strategy, N. ampullaria—the same species in which the tiny frog lays its eggs—grows under the closed forest canopy and has evolved to utilize leaf litter for nitrogen. Its unusual pitcher shape, with the small lid, placed away from the opening, allows falling leaf litter to accumulate inside the pitcher. The various bugs inhabiting the pitcher are thought to play a role in digesting the leaf litter and transferring nitrogen to the plant. Plants taking up nitrogen from the leaf litter exhibit increased growth and photosynthesis.
Pitcher plants need to devote a lot of energy and resources such as carbon to produce pitchers with specialized cells for capturing and digesting prey instead of normal leaves that are specialized to carry out photosynthesis, which can recover the carbon invested in them. As it happens, all the various adaptations of pitchers to trap prey make them less efficient at photosynthesizing: N. alata and N.mirabilis pitchers have lower photosynthetic rates compared to the lamina, the leaf that they hang from. They also have a lower density of stomata, the tiny pores that allow carbon dioxide to enter, than the lamina. Cells that would normally be packed with chloroplasts, where photosynthesis occurs, are replaced with digestive glands, resulting in lower chlorophyll content. Because pitchers are not photosynthesizing as much, they are unable to return the carbon invested in them by the plant, and hence they become a cost to the plant. (More on this in another post).
And because of these costs, when nutrients are abundant or when the soil is too dry, some Nepenthes species forgo producing pitchers altogether, thus saving precious resources. Some Sarracenia species, such as S. flava and S. oreophila, are also part-time carnivores, only producing pitchers in spring.
The evolution of plants with pitfall traps is difficult to decipher because like other carnivorous plants, they do not fossilize well. Also, Nepenethes lacks any close relatives or transitional species. It does, however, have distant relatives: the carnivorous plants Triphyophyllum peltatum, a liana with leaves bearing digestive glands that can absorb nutrients, and Drosophyllum lusitanicum, which is similar to the sticky traps. All Nepenthes species can interbreed with each other, producing fertile offspring.
During the time Nepenthes split from its relatives, the plants shifted to producing either male or female flowers, as opposed to its relatives that produce both. This is rare among flowering plants, and it means that they cannot self-pollinate; there must both a male plant and a female plant to produce seeds. Botanists believe that this contributed to the large number of species concentrated in a small area. It may explain how different mountains in Southeast Asia have their own native Nepenthes species.
Unfortunately, these amazing plants are under numerous threats from human activities. Carnivorous plants are popular among botanists and plant enthusiasts who cultivate them at home. As a result, many species are easily available for sale online. But some species may have been sourced from the wild illegally rather than from artificially propagated plants. Poachers pluck rare species from the wild to engage in this lucrative trade. N. attenboroughii, which is critically endangered, can fetch hundreds of dollars. Unsustainable collection of plants, which in some cases can reach several hundred thousand plants from the wild, is threatening many species. Addressing these concerns, the exact location of some recently discovered species has not been revealed.
Also, because many Nepenthes species are rare, with a narrow range, usually found only in tropical mountainous regions, they require very specific conditions to thrive: high humidity, warm days, and cool nights, with a temperature drop of 10 degrees Celsius between day and night. These conditions may be hard to re-create elsewhere, making them difficult to grow, and many plants may not survive being transplanted elsewhere.
Habitat loss through logging and fires can also contribute to their decline. Along with severe poaching, fires have caused the population of N. clipeata, a critically endangered species since 2000, to plunge 90% in just 30 years. N. clipeata is thought to be the rarest and the most unusual species growing at 700-900m on the granite cliffs of Mount Kelam in Kalimantan, Borneo. In 1997-1998, massive fires, thought to be caused deliberately and which coincided with an El Nino, engulfed the region and destroyed much of their population. Only a few wild plants remained growing in inaccessible locations. Since then, the International Carnivorous Plant Society set up a program to ensure growers can cooperate to ensure artificial propagation.
Pitcher plants may look boring from the outside compared with a Venus flytrap, for example, but as we’ve seen, they have a lot going on inside—and their prey trapping mechanisms are a source of inspiration for researchers. A team of chemists have already created an extremely slippery material mimicking the nectar-lubricated surfaces of pitchers. The coating does not only repel water, but also oil and blood and can be useful in improving fuel transport as well as in devices handling blood, such as kidney dialysis machines.
More recently, a team of researchers, including the previous study author, further developed and tested the coating inside tubes carrying blood. Blood flowing inside tubes can form potentially dangerous clots and there is also the persistent risk that bacteria clinging onto the surfaces of the tubes could cause a nasty infection. Applying the coating to the inside of tubes and catheters, which were then implanted in pigs, they found that blood did not clot for 8 hours, even without blood thinners. Without the coating, blood would clot faster. The researchers claim that such a coating could mean that lower doses of blood thinners may be used in patients, which in turn will lower their risk of bleeding related complications.
The team even tested the surface on geckos, the masters of adhesion. And even for them, the surface proved too slippery; they were unable to grip onto the surface when it was raised to near vertical.
We still have a lot to learn about these amazing plants. And who knows, they may inspire more inventions someday that can improve our lives in ways we would have never imagined.
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John Brittnacher, “Evolution — Nepenthes Phylogeny” International Carnivorous Plant Society
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Except for the paper on fluorescent light prey traps and the two Nature papers, the full text of all the papers are freely available online in case you are interested.